infrared and raman spectra of 4-cyanobenzoic acid on powdered silver

Ž .Vibrational Spectroscopy 21 1999 133–142www.elsevier.comrlocatervibspec

Infrared and Raman spectra of 4-cyanobenzoic acid on powderedsilver

Sang Woo Han, Hyouk Soo Han, Kwan Kim )

Department of Chemistry and Center for Molecular Catalysis, Seoul National UniÕersity, Seoul 151-742, South Korea

Received 30 June 1999; received in revised form 23 August 1999; accepted 30 August 1999

Abstract

Ž .We have attempted to record the infrared and Raman spectra of 4-cyanobenzoic acid 4CBA adsorbed on 2-mm-sizedŽ .silver particles. The diffuse reflectance infrared Fourier transform DRIFT spectrum taken for the sample was little different

Ž .from the reflection–absorption infrared RAIR spectrum taken for the same molecules on vacuum-evaporated thick silverfilms, suggesting that the usual surface selection rule should be applicable even to the surface of fine metal particles. The

Ž .Raman spectrum of 4CBA on powdered silver was a surface-enhanced Raman SER spectrum, exhibiting little differencefrom that taken on vacuum-evaporated thin, rough silver films. Thus, the commercially available powdered silver seemed tobe an efficient substrate for the infrared and Raman spectroscopic characterization of molecular adsorbates prepared in asimilar way on silver surfaces. q 1999 Elsevier Science B.V. All rights reserved.

Keywords: Silver powder; 4-Cyanobenzoic acid; DRIFT; SERS; SEM; STM

1. Introduction

Spectroscopic characterization of molecular ad-sorbates on metal surfaces is very important for thefundamental understanding of various phenomena

w xsuch as heterogeneous catalysis and corrosion 1 .Among the several spectroscopic techniques de-veloped for this purpose, vibrational spectroscopictechniques such as reflection–absorption infrared

Ž .spectroscopy RAIRS and surface-enhanced RamanŽ .scattering SERS are most frequently used for ob-

taining information on structural details of adsor-

) Corresponding author. Tel.: q82-2-880-6651; fax: q82-2-874-3704; e-mail: [email protected]

bates. The sensitivity of SERS is remarkable, en-abling routine investigation of adsorbates even at

w xsubmonolayer coverages 2,3 . SERS has, however,two noticeable disadvantages; namely, that its appli-cability is limited to a few metals and that a set of itssurface selection rules established so far is muchdependent on the specific enhancement mechanism.On the other hand, the applicability of RAIRS is notlimited to a few metals. Moreover, its metal surfaceselection rule demands that only those vibrationalmodes with a component of dynamic dipole moment

w xperpendicular to the surface can be observed 4 . Thedisadvantage of RAIRS is the lower sensitivity com-pared with that of SERS. Hence, the combination ofSERS and RAIRS is expected to be useful in investi-gating molecular adsorption on metal surfaces.

0924-2031r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.Ž .PII: S0924-2031 99 00066-1

( )S.W. Han et al.rVibrational Spectroscopy 21 1999 133–142134

It is desirable that one takes SER and RAIRspectra from the same adsorbatersubstrate system.However, RAIR spectra are usually taken from ad-sorbates adsorbed on flat metal surfaces while SERspectra are obtained either in metal–colloidal media

w xor at roughened-metal electrodes 2 . We have previ-ously prepared a vacuum-evaporated silver film thatenabled us to get the SER as well as RAIR spectra of

Ž . w x4-cyanobenzoic acid 4CBA on silver 5 . Such afilm was obtained by rapidly evaporating silver at10y3 Torr on the chromium-coated glass slide.

Recently, we obtained an infrared spectrum of4CBA adsorbed on fine silver particles with a veryhigh signal-to-noise ratio by diffuse reflectance in-

Ž . w xfrared Fourier transform spectroscopy DRIFTS 6 .The DRIFT spectral pattern was little different fromthe RAIR spectral pattern taken for the same moleculeon a vacuum-evaporated thick silver film. The usualsurface selection rule thus seemed applicable even tothe surfaces of fine metal particles. The latter aspectwas further confirmed by comparing the DRIFT

Ž .spectrum of stearic acid STA on powdered silverwith the RAIR spectrum of STA on a vacuum-

w xevaporated silver film 7 . The temperature depen-dence of the DRIFT spectral pattern was also compa-rable to that of the RAIR spectral pattern.

Since the surface enhancement can occur due todifferent mechanisms, we have been attempting toestablish SER spectral correlation with the adsorp-tion mechanism for a series of related compoundsthrough a detailed analysis of peak shift and bandbroadening caused by surface adsorption. In thatway, we could establish, e.g., a qualitative spectralcorrelation that is applicable to the determination ofthe adsorption mechanism of aromatic nitriles on

w xsilver surface 8–12 . Herein, we report that SER, aswell as DRIFT, spectra can be obtained for 4CBAadsorbed on 2-mm-sized silver particles.

2. Experimental

Silver powders with a nominal particle size ofŽ .2–3.5 mm )99.9% purity were purchased from

Aldrich. Before use, these powders were thoroughlyŽ .washed with absolute ethanol Hayman, )99.9% .

Ž .4CBA Aldrich 99% purity was used as received. Astock solution of 1 mM 4CBA in ethanol, prepared

for the adsorption of the 4CBA molecules on silver,was bubbled with nitrogen before use. All otherchemicals, unless otherwise specified, were reagent-grade.

For the adsorption of 4CBA on powdered silver,0.050 g of silver powder was placed in a clean smallvial into which 0.5 ml of the stock solution wasadded. After 4 h, the liquid phase was decanted. Theremaining solid particles were washed with excessethanol and left to dry in ambient air for 2 h. Thepowdered sample was then transferred to either a

Ž .4-mm diameter cup Harrick microsampling cup forDRIFT spectral analyses or a thin glass capillary for

Ž .Raman SERS spectral analyses.For a comparative study, 4CBA molecules were

also adsorbed on vacuum-evaporated silver sub-strates. The silver substrates were prepared by resis-

Ž .tive evaporation of titanium Aldrich, )99.99%Ž . y6and silver Aldrich, )99.99% at 1=10 Torr on

batches of glass slides, cleaned previously by se-quentially sonicating in isopropyl alcohol, hot 1:3

Ž .H O 30% rH SO , and triply distilled H O. Tita-2 2 2 4 2

nium was deposited prior to silver to enhance adhe-sion of silver to the substrate. To prepare silversubstrates for RAIR spectral analyses, approximately200 nm of silver was slowly deposited on titanium

˚ y1Ž .at a rate of ;2 A s while, for the preparation ofsilver substrates to be used in taking SER spectra,;100 nm of silver was rapidly deposited on tita-

˚ y1Ž .nium at a rate of ;100 A s to yield an appro-priate surface roughness; in fact, the former silversubstrate was SERS-inactive. After the silver sub-strates thus prepared were immersed in a stock solu-tion of 4CBA for 4 h, they were rinsed with excessethanol and then subjected to a strong jet of nitrogengas to blow off any remaining liquid droplets on thesurface. Thereafter, RAIR or SER spectra wererecorded.

Infrared spectra were obtained using a Bruker IFS113v Fourier transform infrared spectrometerequipped with a globar light source and a liquidN -cooled wide-band mercury cadmium telluride de-2

tector. Each RAIR spectrum was obtained by averag-ing 512 interferograms at 4 cmy1 resolution, withp-polarized light incident at 808 on the vacuum-evaporated silver substrate as reported previouslyw x13 . The method for obtaining the DRIFT spectra

w xhas also been reported 6 . The Harrick Model DRA

( )S.W. Han et al.rVibrational Spectroscopy 21 1999 133–142 135

3CI diffuse reflectance accessory was used, and atotal of 128 scans were measured at a resolution of 4cmy1 with the use of previously scanned pure silverpowders as the background. The Happ–Genzel ap-odization function was used in Fourier-transformingall the interferograms. The DRIFT, as well as the

Ž .RAIR, spectra are reported in ylog RrR , where0

R and R are the reflectivities of the sample and the0

bare clean metal substrates, respectively.Raman spectra were obtained using a Renishaw

Model 2000 Raman spectrometer equipped with aholographic notch filter and an integral microscope.The 514.5-nm radiation from a 20-mW air-cooled

Ž .argon ion laser Spectra Physics Model 163-C4210was used as an excitation source. Raman scatteringwas detected with a 1808 geometry using a thermo-electric-cooled CCD detector. The Raman band of asilicon wafer at 520 cmy1 was referenced in cali-brating the spectrometer and the accuracy of spectralmeasurement was estimated to be around 1 cmy1.

Separately, the scanning electron micrograph ofthe powdered silver was taken with a Phillips ModelXL20 scanning electron microscope. The shape ofthe powdered silver was also examined using a

Žscanning tunneling microscope Digital Instruments,.Model Nanoscope IIIa .

3. Results and discussion

w xIn our previous DRIFTS study 6 , silver powderswith diameters greater than 5 mm appeared inappro-priate as adsorbent, probably due to their particlesize being quite close to the wavelength of the lightsource. When 2-mm-sized silver powders were usedas adsorbent, spectra having very high signal-to-noiseratios could be obtained reproducibly for the ad-sorbed species. On this ground, 2-mm-sized silverpowders were exclusively used in the present workas adsorbent of 4CBA molecules. 4CBA moleculesadsorb on the silver surface very favorably. As re-

w xported previously 5 , a complete monolayer is read-ily formed within 1 h by contacting the silver sub-strate with 10y2 M 4CBA in ethanol.

We could obtain DRIFT and RAIR spectra of4CBA on silver with the same quality as that re-ported previously. Fig. 1a and b show the DRIFTand RAIR spectra of 4CBA adsorbed on a powdered

silver and a vacuum-evaporated thick silver film,respectively. For reference, the DRIFT spectrum ofneat 4CBA diluted with KBr powder is shown inFig. 1c. Positions of major peaks appearing in Fig. 1are summarized in Table 1 together with their vibra-tional assignments. Interpretation of the DRIFT spec-trum of 4CBA on powdered silver was provided in a

w xprevious publication 6 . Nonetheless, the main pointsof the earlier interpretation are noted for a betterdiscussion on the SER spectrum. First of all, al-though the DRIFT spectrum of neat 4CBA exhibitscomplex features, only a few peaks are observed inthe DRIFT and RAIR spectra of 4CBA on silver.Also, the latter two DRIFT and RAIR spectral pat-terns are very similar. The C5O stretching band,which appears most distinctly at 1700 cmy1 in theneat spectrum, is completely absent in the adsorbedspectra. This absence, together with the presence ofthe symmetric stretching bands of the COOy groupat 1391 and 1394 cmy1, respectively, in Fig. 1a andb, implies that the adsorption process results in theformation of carboxylate salt. In addition, the totally

Ž .symmetric vibrational bands such as n C[N ,Ž y. Ž .d COO , and n ring mode are also seen in the18a

DRIFT spectrum at 2233, 848, 1020 cmy1 and in theRAIR spectrum at 2233, 849, and 1020 cmy1, re-spectively. The anti-symmetric stretching band of theCOOy group is completely absent in both spectra,however. The present observation indicates that4CBA is as likely to bind to powdered silver as to anevaporated silver film through the carboxylate groupsymmetrically with a perpendicular stance. This ob-servation implies that the usual surface selection ruleis, in fact, applicable to the surfaces of metal pow-ders.

The SER spectrum of 4CBA adsorbed on 2-mm-sized silver particles, shown in Fig. 2a, was obtainedfrom the same sample as that used in obtaining theDRIFT spectrum shown in Fig. 1a. Considering themonolayer coverage of 4CBA molecules, the spec-trum must be a SER spectrum. Fig. 2b shows theSER spectrum of 4CBA adsorbed on a vacuum-evaporated silver film. For reference, the ordinary

Ž .Raman OR spectrum of neat 4CBA is also shownin Fig. 2c. Positions of major peaks appearing in Fig.2 are summarized in Table 1. In fact, the SERspectrum of 4CBA on powdered silver is nearly thesame as that on evaporated silver film. It has to be


Ž . Ž .Fig. 1. a DRIFT spectrum of 4CBA on 2-mm-sized silver powder. b RAIR spectrum of 4CBA on vacuum-evaporated thick silver film.Ž .c DRIFT spectrum of neat 4CBA diluted with KBr powders.

mentioned that the present SER spectral pattern isalso almost the same as the previously reported oneon evaporated silver film for which both the RAIR

and SER spectra have been taken. This implies thatsimilar SER spectral interpretation can be made in

Ž .this work as that reported previously. The n C5O


Table 1Infrared and Raman spectral data and vibrational assignments for 4CBAa

c d eDRIFT RAIR OR SERS AssignmentbNeat Silver powder Silver powder Silver film

yŽ .848 849 851 850 d COO865 5

Ž .932 g OH982 17a

1020 1020 1020 18a1117 18b

1131 1136 1136 7a1185 13q9a

1183 1188 1188 9aŽ . Ž .1290 1285 d OH qn C–O

1323 14yŽ .1391 1394 1395 1394 n COOs

1431 19b1506 19a1568 8a1610 1612 1608 1608 8b

Ž .1700 1632 n C5OŽ .2232 2233 2233 2234 2235 2233 n C[N

3060 3075 3079 3080 20b3102 3085 2

a Wavenumber in cmy1.b Taken in KBr matrix.c RAIRS on silver film.dOR spectrum in solid state.e w x w xDenoted in terms of Wilson 14 notation. Assigned based on Refs. 15,16 .

Ž . Ž .and d O–H qn C–O bands appearing distinctly at1632 and 1285 cmy1 in the OR spectrum of 4CBAŽ . ŽFig. 2c are absent in the SER spectra Fig. 2a and. Ž y.b . Instead, the n COO bands are present at 1395s

and 1394 cmy1 in the SER spectra on powderedsilver and evaporated silver film, respectively. Thisindicates that the molecules adsorb on both surfacesas 4-cyanobenzoate, as concluded from the DRIFTand RAIR spectral features. Since different enhance-ment mechanisms may operate simultaneously, theactual orientation of 4-cyanobenzoate on silver can-not be deduced clearly from the SER spectra, how-ever.

As mentioned in Section 1, we have been attempt-ing to establish a correlation between the SER spec-trum and the adsorption mechanism for a series ofrelated compounds through a detailed analysis ofpeak shift and band broadening caused by surfaceadsorption. In this respect, it is necessary to recall

the vibrational spectrum of 4-cyanobenzoate in a freeŽ y. Ž .state, specifically that the n COO and n CNs

modes are observed at 1382 and 2233 cmy1, respec-tively, in the OR spectrum of 4CBA in a basic

w x Ž y.aqueous medium 5 . This suggests that the n COOs

mode is blue-shifted by as much as 12–13 cmy1

Ž .upon surface adsorption while the n CN mode isŽ y.barely affected. We could also cite that the n COOs

band in the SER spectrum is at least twice as broadas that in the OR spectrum, while the bandwidths of

Ž .the n CN bands in the two spectra are almostidentical. The bandwidths of benzene ring modes arealso hardly different between the OR and SER spec-tra. On these grounds, one can conclude that thenitrile group of 4-cyanobenzoate should be pendentand not take part in the surface adsorption. Thisconclusion agrees with the prediction based on theelectromagnetic surface selection rule. According tothe latter rule, vibrational modes whose polarizability


Ž . Ž . Ž . Ž .Fig. 2. SER spectra of 4CBA adsorbed a on 2-mm-sized silver powder and b on vacuum-evaporated thin silver film see text . cOrdinary Raman spectrum of neat 4CBA.

tensor elements are perpendicular to a metal surfaceshould be much more enhanced in a SER spectrum

w xthan the parallel ones 17 . In this light, the complete

absence of out-of-plane modes in the SER spectrapresented in Fig. 2 may reflect the end-on geometryof 4CBA on silver. In addition, the noticeable ap-


pearance of a band at ;3080 cmy1 in the SERspectra may also imply such an end-on coordinationw x18,19 .

In principle, a carboxylate can be bound to metalvia either the oxygen lone pair electrons or the p

electrons of the carboxylate group. However, a per-pendicular orientation of 4-cyanobenzoate on silversuggests a coordination through the oxygen lone pairelectrons. Recalling that the oxygen lone pair elec-trons of the carboxylate group have an antibonding

w xcharacter 20 , the s donation of the carboxylategroup to silver will lead to strengthening of thecarboxylate group, resulting in the observed blue-shift

Ž y.of the n COO band in Fig. 2. Above descriptionss

do not necessarily mean that the adsorbate shouldassume a perfectly perpendicular orientation, how-ever. In fact, due to the possibility of involvement ofvarious enhancement mechanisms, one can only de-duce from the SER spectral data that 4-cyano-benzoate does not take a flat orientation on the silversurface; it can take either a tilted or a perpendicularorientation. In contrast, the infrared spectral dataclearly dictated that 4-cyanobenzoate should take anearly perpendicular orientation with respect to thesilver surface. An out-of-plane ring mode, such asn , appearing distinctly at 865 cmy1 in a neat state5

was hardly seen in both the DRIFT and RAIRspectra in Fig. 1.

As mentioned in Section 1, the two most commonsubstrates used for the SERS effect are the electro-chemically roughened electrode and metal colloidsw x2 . Many experimental variables are involved in the

w xpreparation of the electrode 21,22 . Although theprocedure for the preparation of metallic solutions issimple, many factors such as time, temperature, con-centration, and impurity can affect the aggregation ofthe colloidal particles, causing substantial intensity

w xvariation in SER spectra 23 . Of course, SERS-ac-tive substrates can also be prepared by vacuum

w xthermal deposition of metals 5,24 or by the chemi-w xcal reduction method 25,26 . The present work

clearly shows that the commercially available pow-dered silver itself is a very efficient SERS-activesubstrate. Any special pre- andror post-treatmentseemed unnecessary for obtaining a high-quality SER

Ž .spectrum at least under a 514.5-nm excitation .More importantly, it has been demonstrated thatSER, as well as DRIFT, spectra can be obtained with

these powders. Considering that the usual infraredsurface selection is applicable even to the DRIFTspectra, the latter aspect must be quite advantageousin the interpretation of SER spectra. In fact, thepossibility of silver powder as a SERS substrate has

w xbeen mentioned in the literature 27–30 , but theadsorption characteristics of adsorbates have not beenconsidered; silver powders have been used only foranalytical purposes.

That both the SER and DRIFT spectra can beobtained for 4CBA adsorbed on powdered silvermay not necessarily mean that the species responsi-ble for the SER spectrum is the same as that for theDRIFT spectrum. One can speculate that certainadsorbed species may not contribute at all to theSER spectrum since the surface enhancement is actu-ally very much dependent on the detailed micro-

Žscopic surface structures related to different crys-. w xtalline planes, defect sites, and adatoms, etc. 31 . In

contrast, all the adsorbed species may contributeŽequally to the DRIFT spectrum, however. In a

separate experiment, we found that 2-mm-sized goldparticles were also very effective for DRIFT spec-troscopy, but unlike the silver particles, they wereSERS-inactive for both the 514.5 and 632.8 nmexcitation. We thus presume that electromagneticenhancement is negligibly involved in the DRIFT

.spectra of 2-mm-sized silver particles. Slightly dif-ferent peak positions observed between the DRIFT

Žand SER spectra of 4CBA on powdered silver see.Fig. 1a, Fig. 2a, Table 1 may thus arise, albeit in

part, from such a different spectral acquisition pro-cess. Nonetheless, the overall adsorption behavior of4CBA on silver observed from the DRIFT spectrumwould be hardly different from that of the SERspectrum.

It is widely accepted that the enhancement of theŽelectromagnetic field near the rough surface EM

.enhancement and the surface resonance Raman ef-Ž .fect chemical enhancement are the main causes of

SERS among several possible mechanisms. In fact,for the EM mechanism to operate, certain degrees ofsurface roughness are required; the chemical en-hancement may also be more effective on atomicallyrough surfaces than on smooth surfaces. Previously,

Ž .we took the scanning electron micrograph SEM ofthe vacuum-evaporated silver substrate that enabled

w xus to record the RAIR and SER spectra 5 . Appar-


ently, the surface of the substrate was much rougherthan that of the silver substrate employed in theusual RAIR experiment. The diameter of silver is-lands on the former substrate was 50–70 nm. Silverislands with either smaller or larger diameters thanthe optimum exhibited significant reduction in SERSenhancement and RAIR intensity. On these grounds,we have taken the SEMs of powdered silver used inthe present work. However, the SEM itself seems to

exhibit the particle size distribution only as shown inFig. 3. The micrometer-sized silver substrates areobviously too large for occurrence of the EM fieldenhancement associated with localized surface plas-

w xmons 32 . Since much smaller microstructures wereexpected to be responsible for the SER spectra of4CBA on powdered silver, we have attempted to seethe atomic scale roughness of the powder silver by

Ž .scanning tunneling microscopy STM . Fig. 4 shows

Ž . Ž .Fig. 3. The SEM photographs of 2-mm-sized silver powder at a 3000= and b 30,000= magnification. Each white marker denotes 10Ž . Ž .mm a and 1 mm b length.


Ž .Fig. 4. The STM image 100 nm=100 nm of 2-mm-sized silver powder.

the STM image for the 2-mm-sized silver powder. Infact, the surface of powdered silver appears to con-sist of a few tens of nanometer-sized protrudes. TheSERS activity of the silver powder can be ascribedto those submicroscale roughness.

4. Summary and conclusion

We have demonstrated for the first time thatDRIFT and SER spectra can be obtained with veryhigh signal-to-noise ratios for 4CBA adsorbed in asimilar way on fine silver particles. The DRIFTspectral pattern was in fact very little different fromthe RAIR spectral pattern taken for the samemolecules on vacuum-evaporated thick silver films.The usual surface selection rule thus seemed applica-

ble even to the surface of fine metal particles. TheSER spectral pattern on the powdered silver was alsofound to show little difference from that onvacuum-evaporated thin, rough silver films. Basedon the STM image, which shows that the surface ofpowdered silver consisted of a few tens of nanome-ter-sized protrudes, the SERS activity of the silverpowder could be ascribed to such submicroscaleroughness enhancing the EM field via localized sur-face plasmon excitations. The commercially avail-able powdered silver seemed to be an efficient sub-strate for the vibrational spectroscopic characteriza-tion of molecular adsorbates on silver surfaces. Thecombination of infrared and Raman spectroscopiesshould be very useful in investigating molecularadsorption on metal surfaces, especially on SERS-ac-tive noble metals like Ag, Au, and Cu.


Acknowledgements

This work was supported by the Korea Scienceand Engineering Foundation through the Center forMolecular Catalysis at Seoul National UniversityŽ .SNU and by the Korea Research Foundationthrough the Research Institute of Basic Sciences atSNU. K.K. appreciates several helpful commentsfrom Professor Hie Joon Kim of the ChemistryDepartment of SNU.

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infrared and raman spectra of 4-cyanobenzoic acid on powdered silver

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